Studies are being carried out to determine whether dietary modification and streptocotocin induced diabetes influence the apparent turnover rate of the component parts of rat liver choline and ethanolamine phosphoglycerides. In rats fed a chow diet the apparent turnover rate of glycerol, in both these lipids, was only about 24 hours. The specific activity of seven molecular fractions, from each of these fractions, at four different time periods was about equal to the molar contribution of that fraction. These results suggest that the unsaturated fatty acid composition does not influence the apparent turnover rate. A variety of different unsaturated acids have been made and used as substrates to determine how structural modification alters the rate at which a fatty acid is chain elongated by rat liver microsomes. Acids with their first double bond at position 7 are the best substrates for chain elongation. The rate of chain elongation is also chain length dependent since 7,10-18:2 was chain elongated better than was 7,10-14:2 or 7,10-16:2 while 7,10-20:2 was a poor substrate.